Characterizing vascular plant and algal community structure and biomass in the Nueces Delta (TX) salt marsh system

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Abstract

Primary producers are responsible for the majority of the services provided by salt marshes, such as nutrient uptake, carbon sequestration, and support for fisheries. The Nueces Delta, a salt marsh in southern Texas, has been severely affected by reduced freshwater inflow, resulting in an altered composition of dominant plant species and functionality of the habitat. This study determined if the primary production along the two main channels in the Nueces Delta was affected differently by restrictions of freshwater inflow. Vegetative and microalgal biomass were harvested monthly at three locations along each channel and correlated to environmental factors. Photosynthetic measurements were also performed to determine stress levels of species present. A concurrent evaluation of government monitoring methods was performed. The study period was marked by rainfall amounts that were double the long-term average and by high vascular plant growth. Vegetative biomass along the Delta Access Channel (422.16 g C m-2) was significantly less (p < 0.005) than that of the Rincon Channel (475.71 g C m-2), with the largest differences in late winter months. Reduced winter vegetation biomass in the Delta Access Channel was accompanied by elevated microalgal biomass compared to the Rincon Channel (p < 0.0001). Maximum quantum yield (Fv/F¬m) of all vascular plants and microalgae remained well below optimum throughout the study and did not differ significantly between channels. Explanatory models revealed warm temperatures and increased freshwater inflow were the most important factors governing vegetative biomass, while microalgae thrived under salinity extremes and cooler temperatures. These results indicate that vegetation in the Delta Access Channel is impacted to a greater extent by reduced freshwater inflow. The RAM protocol currently used by the Texas General Land Office was unable to detect differences between seasons or sites. However, this indicates robustness of the protocol instead of a failure. Normalized difference vegetation index (NDVI) calculated from hyperspectral measurements was able to detect seasonal variation. The lower Nueces Delta should continue to be monitored to verify long term hydrologic and vegetation trends. Water management of the system may require channelization modifications to enhance freshwater flow into the Delta Access Channel and improve overall primary production in the Nueces Delta.